Vascular endothelial growth factor (VEGF) has been identified as a key mediator of tumor angiogenesis and has emerged as the single most commonly upregulated angiogenic factor in both grafted and naturally occurring tumors. VEGF exerts its biological activity by binding to its tyrosine kinase receptors, VEGF-R1 (Flt-1) and VEGF-R2 (Flk-1/KDR). Preclinical studies have examined the biological activities of different types of VEGF antagonists, including anti-VEGF neutralizing antibodies, soluble versions of the receptor molecules, and inhibitors of VEGF-R2 tyrosine kinase. In each case, significant inhibition of tumor growth and reduced vasculature was observed. We propose to prepare and evaluate long acting versions of one of these antagonists, specifically VEGF-R1 receptor (Flt-1). During Phase I we identified sites in the soluble portion of Flt-1 (sFlt-1) that can be modified with a polyethylene glycol moiety without significantly affecting the protein's in vitro bioactivity. In addition, pharmacokinetic studies were performed in rats to verify that the addition of an inert polymer extends the circulating half-life of sFlt-1. During Phase II, we will manufacture sufficient quantities of the modified sFlt-1 proteins for testing in a number of animal models of cancer. Based on our previous work, these modified sFlt-1 proteins should be superior to unmodified preparations of recombinant soluble Flt-1. In particular we expect to see improved stability, higher potency with less frequent dosing, greater solubility and reduced antigenicity. These improved biological and physical characteristics should expedite pre-clinical and clinical evaluation of sFlt-1 as an anti-angiogenesis agent. [unreadable] [unreadable] [unreadable]